As a colored glass plate, a heat absorbing glass for automobiles is, for example, known which is made of alkali-containing silica glass and whereby transmitted light has a blue or green color as it contains iron as a coloring component (Patent Documents 1 to 3). The iron in the glass is present in the form of a divalent or trivalent iron. The divalent iron has an absorption peak at a wavelength in the vicinity of 1,100 nm, and the trivalent iron has an absorption peak at a wavelength in the vicinity of 400 nm. Therefore, in a case where attention is paid to the blue or green color of transmitted light, it is preferred to increase the mass percentage of divalent iron calculated as Fe2O3 in the total iron calculated as Fe2O3 (hereinafter referred to as Fe-Redox).
Further, a colored glass plate made of alkali-containing silica glass usually contains sulfur derived from sodium sulfate (Na2SO4) contained as a refining agent in glass raw material.
In a case where sodium sulfate (Na2SO4) is used, as the amount incorporated increases, the refining effect becomes high, and it becomes easy to remove bubbles during glass melting. However, sulfur derived from sodium sulfate is present in the form of minus divalent or hexavalent sulfur in glass, whereby the minus divalent sulfur has an amber color having an absorption peak at a wavelength in the vicinity of 380 nm, and the hexavalent sulfur is colorless. Therefore, in order to obtain a colored glass plate whereby transmitted light has the desired color, it is preferred to suppress formation of the minus divalent sulfur (amber color development) as far as possible.
Whereas, in order to increase Fe-Redox, it is necessary to add a reducing agent (coke or the like) in a large amount to the glass material. However, at the time of reducing the trivalent iron to the divalent iron by the reducing agent, the hexavalent sulfur is also reduced to minus divalent sulfur by the reducing agent, whereby the amber color development becomes distinct. Therefore, it is difficult to bring Fe-Redox to be at least 60% while suppressing the amber color development.